feat(evals): add skia category with 20 react-native-skia evals

[andriicallstack]

Summary

Adds a new skia eval category with 20 evaluations covering the core @shopify/react-native-skia API surface.

Each eval includes a focused prompt, atomic requirements, and a reference implementation. A category README.md with a best-practice inventory and eval traceability matrix is also included.

Evals added

#	Eval	Focus
01	canvas-fill-background	Canvas, Fill, useCanvasSize
02	shape-primitives	Rect, Circle, RoundedRect, Line
03	path-drawing	Path, Skia.Path.Make()
04	paint-stroke-fill	Paint, stroke vs fill
05	linear-gradient	LinearGradient, vec
06	radial-gradient	RadialGradient
07	image-display	Image, useImage
08	text-rendering	Text, matchFont
09	blur-filter	Blur
10	color-matrix-filter	ColorMatrix
11	reanimated-basic-animation	useSharedValue, withRepeat, withTiming
12	derived-value-animation	useDerivedValue
13	animated-color-interpolation	interpolateColors
14	gesture-pan	GestureDetector, Gesture.Pan
15	transforms	transform, Group
16	clip-rect-and-path	ClipRect, ClipPath
17	blend-mode	blendMode
18	svg-path-rendering	Skia.Path.MakeFromSVGString
19	runtime-effect-shader	Skia.RuntimeEffect.Make, GLSL
20	canvas-snapshot	useCanvasRef, makeImageSnapshot

Runner fixes

• Apply extractJsonMiddleware in both the solver and judge client to handle Claude wrapping JSON responses in markdown code fences (AI_NoObjectGeneratedError)
• Improve ensureOpencodeServerStarted to reuse an already-running OpenCode server instead of attempting a duplicate startup, and forward ANTHROPIC_API_KEY to newly spawned servers

Baseline scores

Solver and judge: anthropic/claude-sonnet-4-5

Score	Evals
100%	01, 02, 04, 06, 07, 09, 10, 11, 12, 13, 16, 17
75%	03, 05, 08, 14, 15, 18, 20
50%	19

Average: ~88%